1. Field of Invention
This invention relates to joints used in the construction of pavement.
2. Description of Related Art
Temperature changes cause materials and structures to contract and expand. This process is the main force responsible for the cracking and premature destruction of pavement. These adverse effects are further accelerated and exacerbated by prolonged exposure to frequent and substantial temperature fluctuations.
To prevent this from happening, longitudinal or transversal joints are employed. These partition the pavement into small fields, which expand and contract independently of each other.
The highest level of stress and deflection in pavement is found at the joints. For highway pavement, the stress is highest along the longitudinal and transversal joints and the deflection is highest at the corners.
Among the most commonly used methods aimed at enhancing the performance of transverse and longitudinal joints are the following:                (i) Increasing the thickness of the slab and base course in order to improve aggregate interlock;        (ii) Protecting the base and sub-grade against water intrusion;        (iii) Installing permeable base materials;        (iv) Reducing joint spacing; and        (v) Installing load transfer devices (e.g., Dowel bars).        
Industry practice and research have determined that the latter technology is vary effective in maintaining load transfer throughout the life of the pavement. Dowel bars' ability to reduce faulting by limiting vertical joint movement further enhances the joint load transfer efficiency. Contraction joints with built-in Dowel bars during original construction normally maintain adequate deflection load transfer (70–100% from the loaded to the unloaded slab).
These contraction joints are employed according to the following procedure:                1. Dowels are placed by welded Dowel assemblies fastened to the base by mechanical means.        2. Concrete is set along the entire width and length of the area under construction.        3. After the concrete has set, it is cut by concrete saws to a 7.5 cm depth and minimum of 3 mm width. The joint is cleaned with compressed air and is filled with sealant material.        
These contraction joints suffer from a number of disadvantages:                1. Uneven process of joints setting.                    Due to the weight of passing vehicles, the pavement, already partially sawn to a 7.5 cm depth, may crack further down, potentially resulting in the pavement being sawn off completely across its entire depth.                        2. Technology requires large amounts of steel.                    Large amount of steel is used for the Dowel bars in each joint. The load from the passing vehicles exerts pressure on the Dowel bars, possibly cutting them. This requires the diameter of the Dowel bars to be as large as 1.5″ (3.8 cm). The length of the Dowel bars is, in most cases, 18″ (46 cm). The Dowel bars are placed symmetrically on both sides of the joint. The Dowel bars carry the load independent of one another. This requires the Dowels bars to be arranged in short distances from one another with a cross section with circumference of 12″ (30 cm).                        3. Technology fails to provide complete and reliable waterproofing.                    The joint is not completely waterproof, a deficiency that is the main reason for the formation of cracks along the joints. The sealant, which is set in hot condition, fills completely the joint space and sticks to the sides. At low temperatures the material becomes brittle (largely due to the vibration motion of the vehicles). The resulting cracks destroy the connection with the sides of the joints. This process enables water to go through the joint and reach the base of the pavement. As a result, the base of the pavement is softened and ultimately deformed. The repeated freezing and thawing of the water in the joints further exacerbates the problem: it leads to failure of the concrete around the joint and to widening of the joint. The water in the base of the pavement makes the base softer and therefore decreases its bearing capabilities.                        4. Method is technologically and economically sub-optimal.                    The Dowel bars need to be placed before further work can be conducted. This prevents concrete trucks from moving along the strip where concrete is to be poured. Concrete trucks must then move outside the lane, requiring valuable space and preventing simultaneous work on multiple lanes. Furthermore, special machines are needed to complete the pouring of the concrete, which results in increased production costs.                        